Methods for detecting and monitoring sleep disordered breathing using an implantable medical device

ABSTRACT

A method of identifying sleep disordered breathing (SDB) in a patient includes monitoring a hemodynamic pressure, deriving high, middle, and low values representative of the distribution of the hemodynamic pressure over a storage interval, measuring a ratio of a lower range to a full range of the hemodynamic pressure based on the derived high, middle, and low values, and using the ratio to determine whether the patient has experienced an SDB episode. Certain embodiments of the invention compare the ratio to a threshold value to identify the occurrence of an SDB episode, while other embodiments of the invention identify the occurrence of an SDB episode by monitoring for a simultaneous increase in both the ratio and the full range of the hemodynamic pressure. In certain other embodiments of the invention, activity level and/or duration criteria may be employed to confirm the occurrence of an SDB episode detected using the ratio.

FIELD

The disclosure relates generally to the fields of heart failuremanagement and sleep disordered breathing (SDB), and more particularly,to a method for screening heart failure patients for the presence ofSDB.

BACKGROUND

Typically, patients with heart failure have a reduced capacity formyocardial function. The heart is unable to adequately meet themetabolic demands of the body by providing the appropriate blood flow.This may result in increased blood pressure (afterload), and increasedvolume retention (preload). Thus, common symptoms of ventriculardysfunction or heart failure include fatigue, which is caused by the lowcardiac output, and edema and swelling, which is caused by fluidoverload.

Current guidelines for the evaluation and management of chronic heartfailure (HF) tend to focus on the presentation of subjects while theyare awake. However, several recent studies have shown that sleepdisordered breathing (SDB), commonly known as sleep apnea, may play animportant role in the pathogenesis and progression of heart failure.

The prevalence of undiagnosed sleep apnea in the U.S. is thought to bein the millions with on the order of 2% of middle-aged women and 4% ofmiddle-aged men having sleep apnea syndrome. See Young T. et al., “Theoccurrence of sleep-disordered breathing among middle-aged adults,” NewEngland J. Med. 1993;328:1230-1235. Sleep apnea is a condition thatresults from a reduction in air intake through the air passage ofsleeping individuals. This problem arises as a result of weak muscletone in the throat and, although compensated for during waking hours,gives rise to symptoms of fatigue during the day, poor quality sleep atnight, and heavy snoring during sleep. Diagnosis of sleep apnea has beencarried out in sleep laboratories where the patient is monitored atnight during sleep in a process called nocturnal polysomnography. Thisdiagnostic test is expensive, time consuming, and must be administeredby highly trained technicians. Consequently, availability of the test islimited.

Sleep disordered breathing (SDB) is estimated to occur in about 60% ofpatients suffering from congestive heart failure (CHF; Rechtschaffen A,Kales A, eds. A Manual of Standardized Technology, Techniques andScoring System for Sleep Stages of Human Subjects. Los Angeles: UCLABrain Information Service/Brain Research Institute, 1968). Nasalcontinuous positive airway pressure (CPAP) has been advocated as anonpharmacological treatment for patients with congestive heart failureand certain forms of SDB.

The clinical implications of SDB are not widely recognized and areseldom taken into account in the evaluation and management of heartfailure (HF). The conventional approach to the evaluation and managementof HF may need to be modified in view of a growing body of evidenceshowing that the acute and chronic mechanical, hemodynamic, autonomic,and chemical effects of SDB place subjects with HF at increased risk ofaccelerated disease progression. A convenient method of screening heartfailure patients for the presence of SDB is therefore desired.

SUMMARY OF THE INVENTION

In certain embodiments of the invention, a method of identifying sleepdisordered breathing (SDB) in a patient includes measuring a hemodynamicpressure parameter, periodically deriving statistical information aboutthe hemodynamic pressure parameter at storage intervals, includingvalues representative of the distribution of the hemodynamic pressureparameter over each storage interval, measuring a ratio of a lower rangeto a full range of the hemodynamic pressure parameter, and using theratio to determine whether the patient has experienced an SDB episode.Further embodiments of the invention include comparing the ratio to aratio threshold, or monitoring for an increase in both the ratio and thefull range of the hemodynamic pressure parameter to identify an SDBepisode.

In other embodiments of the invention, a method of identifying sleepdisordered breathing (SDB) in a patient includes measuring a hemodynamicpressure parameter over a period of time, monitoring an activity levelsignal of a patient over the period of time, measuring a phasedifference between the hemodynamic pressure parameter and activity levelsignals, and identifying the presence of SDB if the phase difference isgreater than a threshold amount.

In certain other embodiments of the invention, a method of identifyingsleep disordered breathing (SDB) in a patient includes measuring ahemodynamic pressure parameter signal over a period of time, monitoringan activity level signal of a patient over the period of time,classifying the activity level as being low when below a predeterminedactivity level, and, during a period of low activity level, identifyingthe presence of SDB when a change in the hemodynamic pressure parametersignal is greater than a specified amount.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an implantable medical device (IMD) thatmay be used in accordance with certain embodiments of the invention.

FIG. 2 is a series of plots of a hemodynamic pressure parameter signaland various other parameters derived therefrom in accordance with anembodiment of the invention.

FIG. 3 is a plot showing the use of short-term and long-term averages todetect increases in a hemodynamic pressure parameter signal inaccordance with certain embodiments of the invention.

FIG. 4 is a plot showing the use of an activity level signal to confirmthe presence of an SDB episode in accordance with an embodiment of theinvention.

FIG. 5 is a plot of a hemodynamic pressure parameter and an activitylevel signal over a period of several days for a patient with an SDBcondition.

FIG. 6 is a flow chart illustrating a method of screening a patient forthe presence of SDB in accordance with an embodiment of the invention.

FIG. 7 is a plot of a hemodynamic pressure parameter and an activitylevel signal.

FIG. 8 is a flow chart illustrating a method of screening a patient forthe presence of SDB in accordance with an embodiment of the invention.

FIG. 9 is a series of plots showing the effect of an SDB therapy onhemodynamic pressure parameters.

FIG. 10 is a flow chart illustrating a method of evaluating theeffectiveness of an SDB therapy in accordance with an embodiment of theinvention.

DETAILED DESCRIPTION

The following detailed description should be read with reference to thedrawings, in which like elements in different drawings are numberedidentically. The drawings depict selected embodiments and are notintended to limit the scope of the invention. It will be understood thatembodiments shown in the drawings and described below are merely forillustrative purposes, and are not intended to limit the scope of theinvention as defined in the claims.

FIG. 1 is a schematic representation of an implantable medical device(IMD) 14 that may be used in accordance with certain embodiments of theinvention. The IMD 14 may be any device that is capable of measuringpressure signals from within a ventricle of a patient's heart, and whichmay further be capable of measuring other signals, such as the patient'selectrogram (EGM). Such a device may be an implantable hemodynamicmonitor (IHM) such as the Chronicle™ device from the MedtronicCorporation. The Chronicle™ and its associated leads and circuitry aredescribed in commonly-assigned U.S. Pat. Nos. 5,535,752; 5,564,434;6,024,704; and 6,152,885, which are incorporated herein by reference intheir entireties. Other pacing systems known in the art may be adaptedfor use in the alternative. The IMD may additionally, or in thealternative, include cardioversion/defibrillation circuitry as describedin commonly-assigned U.S. Pat. Nos. 5,193,535, and 5,314,430, which areincorporated herein by reference in their entireties. Other devices suchas implantable drug delivery devices may also be adapted for use withthe current invention.

The leads and circuitry disclosed in the above-incorporated, commonlyassigned, '752 and '434 patents can be employed to record EGM andabsolute blood pressure values over certain time intervals. The recordeddata may be periodically telemetered out to a programmer operated by aphysician or other healthcare worker in an uplink telemetry transmissionduring a telemetry session.

With continued reference to FIG. 1, the IMD/IHM 14 may be implantedsubcutaneously, between the skin and the ribs. Other implantation sitesmay be used if appropriate. In one embodiment, a lead 12 is passedthrough a vein into the right ventricle of the heart 10. The distal endof the lead or catheter may have a tip electrode 22 contacting theinterior of the heart. In a multipolar configuration, a second ringelectrode 25 may be spaced from the tip electrode 22. Each of theseelectrodes is connected to the circuitry contained in the IMD 14.Alternatively, a unipolar mode may be used wherein a portion of themetallic enclosure or “can” of the IMD may form an electrode surface 24.The EGM signal is measured between this surface and an implantedelectrode such as the tip electrode 22. In yet another embodiment, aSubcutaneous Electrode Array (SEA) such as electrodes 18 and 20 may belocated on, but electrically isolated from, the housing of theimplantable device such as disclosed in U.S. Pat. No. 5,331,966,incorporated herein by reference in its entirety.

Lead 12 is shown to further include a pressure sensor 16. If desired, anadditional lead coupled to IMD 14 may be provided to carry the pressuresensor. The pressure sensor is preferably located within the rightventricle, although it may also be located within the left ventricle.Pressure sensors and accompanying circuitry that may be adapted for usewith embodiments of the invention are described in commonly-assignedU.S. Pat. Nos. 5,353,752, 5,353,800, 5,564,434, 5,330,505, and 5,368,040which are incorporated herein by reference in their entireties.

A typical sleep apnea, or SDB, event is manifested by reduced airflowlasting approximately 10-30 seconds, followed by hyperventilation forapproximately 10-20 seconds. In patients with moderate to severe SDB,SDB events tend to occur in clusters. The apnea-recovery cyclestypically continue many times, and often last more than about 5 minutes.However, the cycles may continue for over an hour, causing oscillationsin physiological parameters such as heart rate, hemodynamic pressures,and saturated oxygen (SaO₂) levels.

The existence of prolonged periods of apnea-recovery cycles may modifyvarious hemodynamic parameters, such as RV pressures, to such an extentthat the changes in hemodynamic parameters may serve as an indication ofan SDB condition.

An implantable hemodynamic monitor (IHM), such as the Medtronic®Chronicle™, may be used for recording a variety of hemodynamicparameters in a HF patient, for example, including right ventricular(RV) systolic and diastolic pressures (RVSP and RVDP), estimatedpulmonary artery diastolic pressure (ePAD), pressure changes withrespect to time (dP/dt), heart rate, activity, and temperature. Someparameters may be derived from others, rather than being directlymeasured. For example, the ePAD parameter may be derived from RVpressures at the moment of pulmonary valve opening, and heart rate maybe derived from information in an intracardiac electrogram (EGM)recording. Hemodynamic pressure parameters may be obtained by using apressure sensor mounted on a lead to measure intracardiac bloodpressures, including absolute and/or relative pressures. U.S. Pat. No.6,865,419 to Mulligan et al., incorporated herein by reference in itsentirety, discloses a method of deriving mean pulmonary arterialpressure (MPAP) from an IHM such as the Chronicle™.

Information collected by an IHM device such as the Chronicle™ can beretrieved and transmitted to an external device, or to a patientmanagement network, or to a database, using various transmission methodsincluding the Internet. For example, a patient may be able to activatethe device to retrieve and transmit the data stored in the IHM to aremote system where additional processing may be performed on the data.This retrieval and transmission of stored data may be done on a periodicbasis, such as once per week, to provide a convenient method of“continuous” monitoring of a patient. Stored data retrieved from the IHMand transmitted to a remote system may be available for transfer to aclinical center for review by a clinician. Data is preferablytransferable to an internet-compatible central patient managementnetwork for remote monitoring. A bi-directional communication systemthat is network, Internet, intranet and worldwide web compatible toenable chronic monitoring based on data obtained from implantablemonitors is generally disclosed in International Publication No. WO01/70103 A2, to Webb et al, incorporated herein by reference in itsentirety.

The data stored by an IHM may include continuous monitoring of variousparameters, for example recording intracardiac EGM data at samplingrates as fast as 256 Hz or faster. In certain embodiments of theinvention, an IHM may alternately store summary forms of data that mayallow storage of data representing longer periods of time. In oneembodiment, hemodynamic pressure parameters may be summarized by storinga number of representative values that describe the hemodynamicparameter over a given storage interval. The mean, median, an upperpercentile, and a lower percentile are examples of representative valuesthat may be stored by an IHM to summarize data over an interval of time(e.g., the storage interval). In one embodiment of the invention, astorage interval may contain six minutes of data in a data buffer, whichmay be summarized by storing a median value, a 94^(th) percentile value(i.e., the upper percentile), and a 6^(th) percentile value (i.e., thelower percentile) for each hemodynamic pressure parameter beingmonitored. In this manner, the memory of the IHM may be able to provideweekly or monthly (or longer) views of the data stored. The data buffer,for example, may acquire data sampled at a 256 Hz sampling rate over a 6minute storage interval, and the data buffer may be cleared out afterthe median, upper percentile, and lower percentile values during that 6minute period are stored. It should be noted that certain parametersmeasured by the IHM may be summarized by storing fewer values, forexample storing only a mean or median value of such parameters as heartrate, activity level, and temperature, according to certain embodimentsof the invention.

During sleep apnea events, RV pressures may decrease due to ineffectiveinspiratory efforts against a closed airway, causing negativeintrathoracic pressure. During a recovery or arousal from an apneaevent, the pressure may increase briefly. This situation is showngraphically in FIG. 2 in the plot labeled PRESSURE (mmHg). The top line,P_(H) 26, is a plot of an upper percentile value (e.g., the 94^(th)percentile) representative of the upper range of a hemodynamic pressureparameter (e.g., RVDP), measured and plotted at periodic storageintervals (e.g., six minute intervals). The bottom line, P_(L) 30, is aplot of a lower percentile value (e.g., the 6^(th) percentile)representative of the lower range of the same hemodynamic pressureparameter over the same storage intervals as plotted for P_(H). The lineplotted between P_(H) 26 and P_(L) 30 is, in this example, the medianvalue of the hemodynamic pressure parameter plotted over the samestorage intervals, and is designated P_(M) 40. It should be noted thatthe choice of the 6^(th) and 94^(th) percentiles as the lower and upperpercentile values is meant to be by way of example only; otherpercentile values could have been chosen to reflect the distribution ofdata without departing from the scope of the invention. Similarly, thechoice of RVDP as the hemodynamic pressure parameter, as well as thechoice of six minutes as the storage interval, and the use of the median(rather than the mean or the mode, for example) for the P_(M) parameter,are all considered exemplary, since other choices for these parameterswould become apparent to one of ordinary skill in the art with thebenefit of these teachings.

With continued reference to FIG. 2, the recurrence of apnea-recoverycycles over a period of time may tend to cause the lower percentilevalue of the hemodynamic pressure parameter, P_(L), to decrease alongwith a corresponding increase in the value of the parameter range,P_(H)−P_(L). The median value, P_(M), tends to remain relativelyunaffected.

According to an embodiment of the invention, an SDB episode may bedetected by calculating a ratio of the lower portion of the parameterdistribution to the full range, P_(Range) 60, of the parameterdistribution, then comparing the ratio to a predetermined thresholdcriterion. This ratio may also be described as the skewness of thesample distribution of the hemodynamic pressure parameter. If the ratio,P_(Ratio) 50, is greater than the threshold, for example, then SDB maybe suspected. One way of performing this ratio calculation is by takingthe difference between the P_(M) 40 and P_(L) 30 values and dividing bythe difference between the P_(H) 26 and P_(L) 30 values, as follows:P _(Ratio)=(P _(M) −P _(L))/(P _(Range))=(P _(M) −P _(L))/(P _(H) −P_(L)),whereP _(Range)=(P _(H) −P _(L)).

In FIG. 2, the P_(Ratio) 50 is plotted as a function of time in thesecond plot, labeled RATIO. As one possibility, a ratio thresholdcriterion 70 could be set to a value of 0.7, for example, and used withthe particular data shown in FIG. 2 to differentiate between SDB andnon-SDB periods. Other values for the ratio threshold criterion 70 maybe derived from historical data or past experience, and may be avariable, user-selectable, programmable setting.

In a further embodiment of the invention, an SDB episode may be detectedby evaluating trends in both the P_(Ratio) 50 and in the P_(Range) 60,P_(H)−P_(L). In one embodiment of the invention, an SDB event may bedetected when the P_(Ratio), (P_(M)−P_(L))/(P_(H)−P_(L)), and theP_(Range), (50 P_(H)−P_(L)), are both increasing. Various methods ofdetermining whether and/or when the P_(Ratio) 50 and/or P_(Range) 60 areincreasing (or decreasing) may be employed. In one embodiment, afast-moving average (i.e., an average based on relatively recent data,also referred to as a short-term rolling average) may be compared to aslow-moving average (i.e., an average that incorporates older data, alsoreferred to as a long-term rolling average) to determine whether theP_(Ratio) 50 or P_(Range) 60 are increasing or decreasing. For example,if a fast-moving average of the P_(Ratio) 50 is greater than thecorresponding slow-moving average, then the P_(Ratio) 50 may be deemedto be increasing. An increase in the P_(Range) 60 could be similarlydetermined. According to an embodiment of the invention, if both theP_(Ratio) and P_(Range) are deemed to be increasing at the same time,then SDB is suspected. The use of short-term and long-term rollingaverages (e.g., 5-point and 30-point rolling averages, respectively) ofa raw signal to detect increases and decreases in the raw signal isillustrated in FIG. 3. Both types of rolling averages tend to smooth theraw signal 80 to some extent. However, the short-term average 90 tendsto track the raw signal 80 more closely, while the long-term average 100tends to respond more slowly to changes in the raw signal 80.

Other methods for determining when a signal is increasing may beequivalently employed. An alternate method of determining when theP_(Ratio) 50 and/or P_(Range) 60 are increasing may, for example,include calculating the derivatives of the signals and evaluatingwhether they are positive. The methods described are by way ofillustration, and not limitation, as other methods of determiningwhether a signal is increasing will be apparent to those having ordinaryskill in the art.

In certain other embodiments of the invention, an activity level signalmay also be measured and recorded by an IHM and used in conjunction withother indications of sleep apnea, as a way to confirm the presence of anSDB episode. For example, an activity signal may be acquired by anactivity sensor to provide a measure of the activity level of thepatient. In a true SDB episode, the patient should be asleep, whichshould be associated with a low activity level. If the presence of SDBis suspected based upon the aforementioned ratio and/or rangecalculations discussed above, the activity level (possibly measured interms of “activity counts”) measured during the same period of time asthe ratio and range calculations may be used to confirm the presence ofSDB. This confirmation step may involve comparing the measured activitylevel to a predetermined activity threshold value, and evaluatingwhether it is below the threshold. A “low” activity level (i.e., asensed activity level below the predetermined activity threshold) couldbe used to confirm that an SDB episode detected by the above-describedratio and range calculations using hemodynamic pressure parametersactually occurred during periods of sleep. An activity sensor maytherefore be used as a cross-check to verify that a detected SDB orsleep apnea episode occurs when the patient is at a resting level ofactivity indicative of a sleep condition.

To produce the activity level signal, an activity sensor may beincorporated as a piezoelectric element sensitive to body movements suchthat a signal from the activity sensor is correlated to the level of apatient's activity. An accelerometer may also be used to detect when apatient moves or is otherwise physically active. The use of activitysensors is known in the art of rate-responsive pacemakers, and may beimplemented as generally disclosed in commonly assigned U.S. Pat. No.5,052,388 to Sivula, et al., incorporated herein by reference in itsentirety. Alternative implementations of activity sensors for use inrate-responsive pacemakers are generally disclosed in U.S. Pat. No.4,428,378 to Anderson; U.S. Pat. No. 4,896,068 to Nilsson; U.S. Pat. No.4,869,251 and to Lekholm et al., all of which are incorporated herein byreference in their respective entireties.

FIG. 4 illustrates the use of an activity level signal 110 to confirmthe presence of an SDB episode detected by identifying an increase inboth the P_(Ratio) 50 and P_(Range) 60. In the embodiment of theinvention illustrated in FIG. 4, an activity level threshold 120 is setto 0.5 activity counts such that an activity level signal 110 below 0.5activity counts confirms a sleeping condition.

Additionally, other methods for detecting when a patient is likely to beasleep are known for use in cardiac rhythm management devices, and mayprovide a basis for confirming the presence of an SDB episode. Suchmethods may be based on one or more sensor inputs in conjunction with areal-time clock. Sensor signals that may be used for detecting asleeping state may include an activity sensor, a respiration sensor, aposture sensor, a blood temperature sensor, etc. An implantablemulti-axis position and activity sensor is disclosed in U.S. Pat. No.5,233,984, issued to Thompson, incorporated herein by reference in itsentirety. A device capable of determining when a patient is likely to beasleep is disclosed in U.S. Pat. No. 5,630,834, issued to Bardy and U.S.Pat. No. 5,814,087 issued to Renirie, both incorporated herein byreference in its entirety. The devices and methods for determining whena patient is likely to be asleep may be used to confirm the presence ofSDB episodes detected by the use of hemodynamic pressure parametersalone.

A further embodiment of the invention may include a confirmation stepthat may further increase the specificity of detection of SDB episodesby requiring that the detection of SDB persist for a specified duration.For example, in an embodiment of the invention in which the hemodynamicpressure parameter ratio and range calculations, as well as the activitylevel, are all required to meet specified criteria to detect SDB (e.g.,P_(Ratio) and P_(Range) both increasing and activity level less than anactivity level threshold), this confirmation step could further requirethat detection be maintained or sustained for at least X out of the lastY consecutive samples, according to one embodiment of the invention. Oneparticular version of this confirmation step may, for example, includesatisfying the detection criteria for a certain number, X, ofconsecutive samples, or for a certain sustained period of time, T. InFIG. 4, for example, a sustained period of 5 minutes is required inorder to detect an SDB episode.

FIG. 5 illustrates patterns of recorded data for a patient presentingwith SDB. FIG. 5 includes plots of a hemodynamic pressure parameter andan activity level signal over a period of several days superimposed onthe same graph. The hemodynamic pressure parameter may be rightventricular systolic pressure (RVSP), right ventricular diastolicpressure (RVDP), or any other hemodynamic parameter that may be measuredand recorded, including parameters derived from other measuredparameters. The activity level (activity counts) typically indicates adaily circadian pattern, showing a pattern of relatively high activitycounts during the day when the patient is awake, and relatively lowactivity counts at night when the patient is expected to be asleep. Thehemodynamic pressure parameters for a “normal” patient (i.e., a patientwithout SDB) tend to generally follow the pattern of activity level,resulting in a degree of correlation between the two signals that causesthem to appear “in phase” with each other. This “normal” hemodynamicpressure profile is generally represented by the dashed line in FIG. 5,which roughly tracks the activity level signal and which generallycorresponds to a daily circadian pattern. By contrast, patients with SDBtend to present with the paradoxical pattern shown in FIG. 5, where thehemodynamic pressure parameter (indicated by the darkened line) andactivity level signals appear to have a “phase difference” between themthat, in some cases, may be at or near 180 degrees “out of phase.”Additionally, the hemodynamic pressure parameter may exhibit largefluctuations at night during periods of relatively little activity.

A measure of the correlation between the blood pressure parameter andthe activity level signal (e.g., a correlation coefficient) can be usedto quantify the phase difference between the two signals. The measure ofcorrelation may be used to identify the presence of SDB, for example, ifthe correlation is less than a predetermined threshold. This measurementmay also be used to quantify the severity of SDB.

FIG. 6 is a flow chart that illustrates a method of screening a patientfor the presence of SDB based on a measured phase difference between ahemodynamic pressure parameter and an activity level signal. The methodillustrated in FIG. 6 includes the following steps:

Step 1: Obtain signals related to a hemodynamic pressure parameter andto an activity level, and record the signals as a function of time. Thehemodynamic pressure parameter may be the right ventricular diastolicpressure (RVDP), for example. In certain embodiments of the invention,the median value of the RVDP over a storage interval is recorded as afunction of time for comparison to the activity level signal. Theactivity level signal may also be a median value over each storageinterval, or may alternately be a mean value or other similarrepresentative value for each storage interval.

Step 2: Measure the Phase Difference between the hemodynamic pressureparameter and activity level signals. The phase difference may bemeasured in terms of a “phase angle” difference (i.e., a number ofdegrees, with a 180 degree difference indicating two signals completelyout of phase). Alternately, the phase difference between the two signalsmay be quantified by using various measures of correlation (e.g., acorrelation coefficient). In one embodiment of the invention, a methodof quantifying the phase difference between two signals includescalculating and comparing the first derivatives of both signals, andevaluate how frequently the derivatives have the same polarity (e.g., asa percentage). For example, two signals that are out of phase may havefirst derivatives that are of the same polarity for a relatively smallpercentage of the time.

Step 3: Determine whether the measured Phase Difference is greater thana predetermined threshold amount, Z. A predetermined threshold, Z, maybe selected or chosen from historical data, for example, and may beadjusted according to certain embodiments of the invention toappropriately identify the existence of sleep apnea for a given patient.

Step 4: If the Phase Difference is greater than Z, sleep disorderedbreathing (SDB) is suspected. If the Phase Difference is less than Z,SDB is not suspected, and monitoring of the hemodynamic pressureparameter and activity level signals may be continued.

A threshold amount of phase difference may be selected to indicate thepresence of SDB. For example, if the measured phase difference, K, isgreater than a predetermined threshold, Z, then SDB is suspected. If thephase difference is measured in degrees, for example, the threshold, Z,may be chosen such that SDB is indicated by a phase angle difference of180 degrees plus or minus a certain range. Alternately, the threshold,Z, may be defined in terms of a correlation coefficient (e.g., less than−0.5), or a p-value below a specified level, or the first derivatives ofthe two signals having the same polarity less than 25% of the time, forexample.

Hemodynamic pressure parameters that may be used in accordance withvarious embodiments of the invention include parameters that aredirectly measured, such as RVDP and RVSP, as well as parameters that maybe derived from other pressure parameters, such as estimated pulmonaryartery diastolic pressure (ePAD), rate of pressure change (dP/dt), etc.

Another method of screening for the presence of SDB involves detectingthe presence of large changes in a hemodynamic parameter during periodsof low activity level corresponding to a sleep state. FIG. 7 illustratesan example of a hemodynamic pressure parameter (in this example, the6^(th) percentile RVDP signal) plotted along with an activity levelsignal over a period spanning about 6 days. The 6^(th) percentile RVDPsignal in FIG. 7 exhibits a large amount of variability during periodswhen activity level is nearly zero. According to one embodiment of theinvention, an SDB episode may be detected when 1) Activity Level is Low(i.e., activity counts are below a specified level) AND/OR a normalsleep period is occurring (e.g., from about Midnight to about 6:00a.m.), AND 2) the RVDP lower percentile value drops below a specifiedlevel (typically less than about 0 mm Hg).

FIG. 8 is a flow chart that illustrates a method of screening a patientfor the presence of SDB based on the above-described decrease in thehemodynamic pressure parameter. The method illustrated in FIG. 8includes the following steps:

Step 1: Obtain signals related to a hemodynamic parameter (such as bloodpressure) and to an activity level, and record the signals as a functionof time.

Step 2: Process the activity level signal to identify Sleep portions ofa Sleep-Wake cycle. This step may involve further requiring that both a)activity level is low (e.g., activity counts are below a threshold levelfor a certain period of time), AND b) that the period of time is duringnormal sleeping hours (e.g., between Midnight and 6:00 a.m.). It shouldbe noted that certain embodiments of the invention could provide for theidentification of a sleep period when either of the above conditions aremet (e.g., an OR condition). It may further be desirable to allow for auser to select between AND and OR combinations, depending onpatient-specific factors, for example.

Step 3: For each identified Sleep portion, compare the hemodynamicparameter to a threshold value. If the threshold is exceeded, SDB issuspected; if not, monitoring continues. The threshold value may, forexample, be a value that the hemodynamic pressure parameter mustdecrease below in order to cause SDB to be suspected. Alternately, thethreshold may be a specified amount of decrease in the hemodynamicpressure parameter (i.e., a “delta”) that must occur within a certainamount of time in order to suspect an SDB condition.

Evaluating Effectiveness of SDB Therapy.

In patients who have been diagnosed with SDB, a system such as theMedtronic® Chronicle™ system can also be used to monitor theeffectiveness of SDB therapy. For example, continuous positive airwaypressure (CPAP) is one form of SDB therapy. CPAP attempts to reduceventricular pressure variability associated with SDB by alleviating SDB,and by preventing intrathoracic pressure from falling. FIG. 9illustrates the effect that CPAP therapy may have on pressure parametersof interest. Successful CPAP therapy may, for example, cause a decreasein the measured pressure ranges, as shown in FIG. 9. These new, lowerpressure ranges may be used to define a “therapy baseline” (or therapyrange) indicative of a desired outcome. If the measured pressuressubsequently deviate from an established therapy baseline, a physicianor other health care provider may suspect that either the patient is notusing the CPAP therapy (a patient compliance issue), or that the CPAPmay be ineffective for other reasons, such as an inappropriate pressuresetting, or other problems with the CPAP equipment.

FIG. 10 is a flow chart that illustrates a method of evaluating theeffectiveness of an SDB therapy, such as CPAP, on a given patient. Themethod illustrated in FIG. 10 includes the following steps:

Step 1: Begin treatment for SDB, using a therapy such as CPAP.

Step 2: Obtain signals related to a hemodynamic parameter (such as bloodpressure) and to an activity level, and record the signals as a functionof time.

Step 3: Establish a therapy baseline for the hemodynamic parameter byobserving and recording the hemodynamic parameter during successfulapplication of an SDB therapy, and selecting values representative ofacceptable limits for successful SDB therapy.

Step 4: Monitor for a deviation from the established therapy baseline.This step may involve various “change detection” techniques. Changedetection may be accomplished by using a comparison of short- and/orlong-term averages, as discussed above, or by using a cumulative sum(CUSUM) type of change detector, for example.

Thus, embodiments of a METHOD FOR DETECTING AND MONITORING SLEEPDISORDERED BREATHING USING AN IMPLANTABLE MEDICAL DEVICE are disclosed.One skilled in the art will appreciate that the invention can bepracticed with embodiments other than those disclosed. The disclosedembodiments are presented for purposes of illustration and notlimitation, and the invention is limited only by the claims that follow.

1. A method of identifying sleep disordered breathing (SDB) episodes ina patient, the method comprising: measuring a hemodynamic pressure ofthe patient; periodically deriving statistical information about themeasured hemodynamic pressure at storage intervals, the statisticalinformation including a high value, a middle value, and a low valuerepresentative of the distribution of the hemodynamic pressure over eachstorage interval; determining a ratio of a lower range to a full rangeof the hemodynamic pressure, the lower range being equal to a differencebetween the middle value and the low value, and the full range beingequal to the difference between the high value and the low value; anddetermining whether the patient has experienced an SDB episode basedupon the ratio.
 2. The method of claim 1, wherein the hemodynamicpressure is right ventricular diastolic pressure.
 3. The method of claim1, wherein the high, middle and low values correspond to predeterminedpercentile values of the hemodynamic pressure derived over each storageinterval.
 4. The method of claim 3, wherein the predetermined percentilevalues of the hemodynamic pressure are the 94^(th), 50^(th), and 6^(th)percentile value over each storage interval.
 5. The method of claim 1wherein the middle value is selected from the group consisting of amedian, mean, and mode of the hemodynamic pressure derived over eachstorage interval.
 6. The method of claim 1 wherein determining whetherthe patient has experienced SDB episodes comprises: comparing the ratioto a predetermined ratio threshold and identifying when the ratioexceeds the predetermined ratio threshold.
 7. The method of claim 6wherein the predetermined ratio threshold is greater than 0.5.
 8. Themethod of claim 6 wherein determining whether the patient hasexperienced SDB episodes further comprises: monitoring an activity levelof the patient and identifying when the activity level exceeds apredetermined activity threshold.
 9. The method of claim 8 wherein theactivity level of the patient is obtained using one of a piezoelectricsensor and an accelerometer.
 10. The method of claim 6 wherein the stepof using the ratio to determine whether the patient has SDB episodesfurther comprises: determining whether the ratio exceeds thepredetermined ratio threshold for a specified duration.
 11. The methodof claim 10 wherein the specified duration comprises an interval oftime.
 12. The method of claim 10 wherein the specified durationcomprises at least X out of Y storage intervals.
 13. The method of claim1 wherein determining whether the patient has SDB episodes comprises:identifying when both the ratio and the full range of the hemodynamicpressure are increasing.
 14. The method of claim 13 wherein an increasein the ratio and full range of the hemodynamic pressure is determined bycomparing a short-term average of the hemodynamic pressure to along-term average of the hemodynamic pressure.
 15. The method of claim13 wherein an increase in the ratio and full range of the hemodynamicpressure is determined by computing a derivative of the ratio and fullrange.
 16. A method of identifying sleep disordered breathing (SDB) in apatient, the method comprising: measuring a hemodynamic pressure over aperiod of time; monitoring an activity level signal of a patient overthe period of time; measuring a phase difference between the hemodynamicpressure and activity level signals; and identifying the presence of SDBif the phase difference is greater than a threshold amount.
 17. Themethod of claim 16 wherein the phase difference is measured by acorrelation coefficient.
 18. The method of claim 16 wherein the phasedifference is measured by comparing a derivative of the hemodynamicpressure to a derivative of the activity level signal and identifyingwhen the derivatives are of the same polarity.
 19. A method ofidentifying sleep disordered breathing (SDB) in a patient, the methodcomprising: measuring a hemodynamic pressure signal over a period oftime; monitoring an activity level signal of a patient over the periodof time; classifying the activity level as being low when below apredetermined activity level; and during a period when activity level isclassified as low, identifying the presence of SDB when a change in thehemodynamic pressure signal is greater than a threshold.
 20. The methodof claim 19 wherein the change in the hemodynamic pressure signal is adecrease of greater than a predetermined threshold.